Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper.
A pulp mill converts wood chips or other plant fibre source into a thick fibre board (market pulp) which can be shipped and traded as paper-grade or dissolving-grade pulp. Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods (e.g. kraft and sulfite processes). The finished product may be either bleached or non-bleached, depending on the customer requirements.
Wood and other plant materials used to make pulp contain three main components (apart from water): cellulose, lignin and hemicelluloses. The aim of pulping is to break down the bulk structure of the fibre source, be it chips, stems or other plant parts, into the constituent fibres. Chemical pulping achieves this by degrading most part of the lignin and to a different extent hemicelluloses into small, water-soluble molecules which can be washed away from the cellulose fibres while controlling the extent of cellulose degradation. The various mechanical pulping methods, such as groundwood (GW) and refiner mechanical pulping (RMP), physically tear the cellulose fibres from each other. Much of the lignin remains adhering to the fibres. There are a number of related hybrid pulping methods that use a combination of chemical and thermal treatment to begin an abbreviated chemical pulping process, followed immediately by a mechanical treatment to separate the fibres. These hybrid methods include thermomechanical pulping, also known as TMP, and chemithermomechanical pulping, also known as CTMP. The chemical and thermal treatments reduce the amount of energy subsequently required by the mechanical treatment, and also reduce the amount of strength loss suffered by the fibres.
Dissolving pulp or dissolving-grade pulp is a chemical bleached pulp with a high cellulose content enough to be suitable for the production or regenerated cellulose and cellulose derivatives. Dissolving pulp has special properties, such as a high level of brightness and uniform molecular-weight distribution. Dissolving pulp is manufactured for uses that require a high chemical cellulose purity, and particularly low hemicellulose content, since the chemically similar hemicellulose can interfere with subsequent processes. Dissolving pulp is so named because it is not made into paper, but dissolved either in a solvent or by derivatization into a homogeneous solution, which makes it completely chemically accessible and removes any remaining fibrous structure. Once dissolved, it can be spun into textile fibers (such as viscose or Lyocell), or chemically reacted to produce derivatized celluloses, such as cellulose triacetate, a plastic-like material formed into fibers or films, or cellulose ethers such as methyl cellulose, used as a thickener.
An object of the present invention is to upgrade paper-grade pulp (unbleached or partially bleached or fully bleached or bleached market pulp) by removal of hemicelluloses e.g. into dissolving-grade pulp using a combination of enzyme treatment, hot caustic extraction (HCE) and optionally one or more bleaching steps.
HCE has previously only been used as a purification process for sulphite-based production of dissolving pulps and has been considered to not contribute much to the purity of pulps produced from alkaline cooking processes, such as soda and kraft. The other existing alkaline purification process is cold caustic extraction (CCE) which is operated close to room temperature (<40° C.) and at very high sodium hydroxide concentration (1.2-3.0 M equivalent to 5-12% w/w in the liquid phase), while the hot purification process (HCE) is usually run at 70-130° C. and at low NaOH concentration (0.1-0.4 M equivalent to 0.4-1.4% w/w in the liquid phase and typically <0.25 M equivalent to <1.0% w/w in the liquid phase).
The present invention enables the use of HCE as a purification process in the fiberline of an alkaline based pulping process for removal of hemicelluloses e.g. for the production of dissolving pulp through the combined use of a prior enzymatic-stage with hemicellulases.
WO9816682 A2 discloses a process for upgrading paper-grade wood pulp to dissolving-grade pulp by using caustic extraction and xylanase treatments in combination in different steps. However, the concentration range of NaOH disclosed in WO9816682 A2 is very high ranging from 8-12% w/w which is within the same NaOH dosage range as carried out in cold caustic extraction (CCE) but using a non-conventional high temperature of 50-100° C.
The combination of enzyme-treatment with hemicellulases and hot caustic extraction (0.03 g NaOH/g pulp, 80° C., 1 h, 2.5% pulp consistency) was studied by Christov and Prior 1994 (Appl Microbiol Biotechnol 42:492-498) but for acid sulphite pulps and using lower NaOH concentration (0.02M) at low consistency.
In the present invention, the use of an enzyme-stage with hemicellulases can activate the alkaline pulp, such as kraft pulp, for the alkaline purification process in the HCE-stage. The hemicellulases will generate a significant amount of new reducing end groups in the hemicelluloses which in turn can trigger alkaline endwise peeling reactions under the high temperature and alkalinity conditions that can be found in the following HCE-stages.